This invention relates to a remote sensor for detecting and measuring the presence of gas in drilling mud and for measuring the flow velocity of the drilling mud.
In drilling an oil well a drilling fluid known as drilling "mud", is circulated through the drill stem and the bit and into the bore hole to cool and lubricate the bit, to remove drilling debris from the borehole, and to prevent material from entering the well through the bore hole wall. The pressure and the density of the drilling mud is carefully controlled to balance or slightly overbalance the hydrostatic pressure of the earth formations surrounding the bore hole, thereby preventing any fluids which may be trapped in these surrounding earth formations from entering the bore hole. There exits a danger, however, that the drill may enter an earth formation in which abnormally high geopressures are present. In such a situation, the geopressures of the formaton will overbalance the opposing hydrostatic pressure of the drilling mud, thereby permitting the entrapped fluids in the formation to enter the well. Such an occurrence is termed a "kick". If this situation is not detected and controlled promptly, the drilling mud may be substantially displaced and the highly pressurized fluids may flow freely up the well causing what is known as a "blowout".
A particularly dangerous situation occurs when a drill strikes a formation containing abnormally highly pressurized gas. In such a situation minute bubbles of gas will enter the bore hole of the well and rise up through the mud, expanding as they rise due to the decreasing head of mud above them. As the bubbles expand in the mud, they further reduce the hydrostatic head of the mud column allowing more rapid entry of gas bubbles at the bottom. This situation is cumulative, and, if left unchecked, the hydrostatic pressure of the mud will be reduced to such an extent that there will be no longer any effective barrier against the inrushing gas, and a blowout will result.
The prevention of blowouts is most effectively achieved by promptly detecting the influx of highly pressurized fluid into the well before an appreciable amount of drilling fluid is displaced from the well. It is well-known that by measuring certain properties of the mud returning out of the well, the presence of conditions at the bottom of the well which indicate a blowout potential can be detected. For example, the mud returning from the well to the surface may be analyzed by means of conventional catalytic or "Hot Wire" gas detectors to determine the amount of gas in the mud, known as "gas cutting" of the mud. A relatively sudden and substantial increase in the degree of gas cutting indicates that the drill has struck a formation containing abnormally highly pressurized gas. This would be indicated, for example, by a decrease in the volumetric or mass ratio of mud to gas in the mud returning from the borehole. Additionally, the velocity of returning mud can be measured and compared with the velocity of the mud entering the well. A substantial increase in the velocity of the returning mud with respect to the velocity of the mud entering the well likewise indicates the occurrence of a kick.
While several methods have been discovered for taking measurements for gas cutting and mud flow velocity, such methods contemplate the taking of these measurements at or near the earth's surface or, in the case of off-shore wells, on or near the drilling platform. This presents a significant delay between the time the kick occurs and the time that the measurements can be taken to determine the occurrence of the kick due to the time it takes for the mud to travel from the bottom of the borehole to the location of the measuring apparatus. In relatively shallow wells this delay will only be a few minutes, giving the well operator in most cases sufficient time to take corrective action to prevent the blowout. Such is usually the case, for example, in offshore wells operating in water depths of less than about 30 meters. However, the search for oil has led drilling teams to deeper parts of the ocean where water depths may be several hundred to thousands of meters. In such deep offshore wells, it may take as long as one hour or more for the mud to travel from the ocean floor to the ocean surface. There is, therefore, a very long delay between the time a kick occurs at the bottom of the borehole and the time that the occurrence of the kick can be determined by analyzing the mud at the surface, and this long delay may prevent the operator from taking corrective measures before a blowout occurs.
The present invention remedies this problem by providing means for measuring mud/gas mass ratio and mud flow velocity at the point where the borehole enters the ocean floor. The data obtained from these measurements are processed and transmitted via a data link to a monitoring and control console on board the drilling platform. In this manner, the time delay presented by the traveling of the mud from the ocean bottom to the surface is eliminated, giving the drill operator a much quicker warning of an impending blowout.